"Double-tracking" Characteristics of the Spectral Evolution of GRB 131231A: Synchrotron Origin?

• Published in: The Astrophysical journal Vol. 884; no. 2; pp. 109 - 117
• Main Authors: Li, Liang, Geng, Jin-Jun, Meng, Yan-Zhi, Wu, Xue-Feng, Huang, Yong-Feng, Wang, Yu, Moradi, Rahim, Uhm, Z. Lucas, Zhang, Bing
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20.10.2019; IOP Publishing
• Subjects: Astrophysics; Emission; Emitters; Energy flux; Evolution; Gamma ray bursts; Gamma rays; gamma-ray burst: general; Photosphere; Radiation; radiation mechanisms: non-thermal; Spectra; Tracking
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.3847/1538-4357/ab40b9

The characteristics of the spectral evolution of the prompt emission of gamma-ray bursts (GRBs), which are closely related to the radiation mechanism (synchrotron or photosphere), are still an unsolved subject. Here, by performing the detailed time-resolved spectral fitting of GRB 131231A, which has a very bright and well-defined single pulse, some interesting spectral evolution features have been found. (i) Both the low-energy spectral index and the peak energy Ep exhibit the "flux-tracking" pattern ("double-tracking" characteristics). (ii) The parameter relations, i.e., F (the energy flux)- , F-Ep, and Ep- , along with the analogous Yonetoku Ep-Lγ,iso relation for the different time-resolved spectra, show strong monotonous (positive) correlations, both in the rising and the decaying phases. (iii) The values of do not exceed the synchrotron limit ( = −2/3) in all slices across the pulse, favoring the synchrotron origin. We argue that the one-zone synchrotron emission model with the emitter streaming away at a large distance from the central engine can explain all of these special spectral evolution characteristics.